Active intestinal chloride secretion in human carriers of cystic fibrosis mutations: an evaluation of the hypothesis that heterozygotes have subnormal active intestinal chloride secretion

CFTR function is impaired in a subset of patients with pancreatitis carrying rare CFTR variants

BACKGROUND

Many affected by pancreatitis harbor rare variants of the cystic fibrosis (CF) gene, CFTR, which encodes an epithelial chloride/bicarbonate channel. We investigated CFTR function and the effect of CFTR modulator drugs in pancreatitis patients carrying CFTR variants.

METHODS

Next-generation sequencing was performed to identify CFTR variants. Sweat tests and nasal potential difference (NPD) assays were performed to assess CFTR function in vivo. Intestinal current measurement (ICM) was performed on rectal biopsies. Patient-derived intestinal epithelial monolayers were used to evaluate chloride and bicarbonate transport and the effects of a CFTR modulator combination: elexacaftor, tezacaftor and ivacaftor (ETI).

RESULTS

Of 32 pancreatitis patients carrying CFTR variants, three had CF-causing mutations on both alleles and yielded CF-typical sweat test, NPD and ICM results. Fourteen subjects showed a more modest elevation in sweat chloride levels, including three that were provisionally diagnosed with CF. ICM indicated impaired CFTR function in nine out of 17 non-CF subjects tested. This group of nine included five carrying a wild type CFTR allele. In epithelial monolayers, a reduction in CFTR-dependent chloride transport was found in six out of 14 subjects tested, whereas bicarbonate secretion was reduced in only one individual. In epithelial monolayers of four of these six subjects, ETI improved CFTR function.

CONCLUSIONS

CFTR function is impaired in a subset of pancreatitis patients carrying CFTR variants. Mutations outside the CFTR locus may contribute to the anion transport defect. Bioassays on patient-derived intestinal tissue and organoids can be used to detect such defects and to assess the effect of CFTR modulators.

Trypanosoma cruzi trans-sialidase induces STAT3 and ERK activation by prokineticin receptor 2 binding

Tc85, as other members of trans-sialidase family, is involved in Trypanosoma cruzi parasite adhesion to mammalian cells. Particularly, Tc85 acts through specific interactions with prokineticin receptor 2, a G-protein coupled receptor involved in diverse physiological and pathological processes. In this manuscript, through biochemical analyses, we demonstrated that LamG, a Tc85 domain, physically interacts with the prokineticin receptor 2. Moreover, expressing prokineticin receptor 1 and 2 we demonstrated that LamG specifically activates prokineticin receptor 2 through a strong coupling with G_αi or G_αq proteins in yeast strains and inducing ERK and NFAT phosphorylation in CHO mammalian cells. To demonstrate a Tc85 physiological role in T. cruzi infection of the nervous system, we evidenced a strong STAT3 and ERK activation by LamG in mice Dorsal Root Ganglia. L173R is the most common prokineticin receptor 2 mutation reported in Kallmann syndrome and it is a founder mutation. Our results demonstrated that in cells co-expressing prokineticin receptor 2 mutant (L173R) and wild-type, LamG is unable to induce signal transduction. The L173R mutation in heterozygosity may allow for a selective advantage due to increased protection from T. cruzi infection. SIGNIFICANCE OF THE STUDY: The Chagas' disease affecting millions of people worldwide is caused by an eukaryotic microorganism called T. cruzi. Pharmacological treatment for patients with Chagas' disease is still limited. Indeed, the small number of drugs available shows important side effects that can be debilitating for patient health. In order to replicate and produce new parasites T. cruzi uses a complex of different proteins produced by both the parasite and the human host cells. So, understanding the molecular details used by T. cruzi to be internalised by different types of human cells is an important step towards the development of new drugs for this disease. Prokineticin receptors are relevant for host-parasite interaction. To characterise the signal transduction cascade induced by their activation may help to understand the molecular details of cell infection, leading to novel therapeutic alternative for this debilitating disease.

Re-imagining cystic fibrosis care: next generation thinking

Cystic fibrosis (CF) is a common multi-system genetically inherited condition, predominately found in individuals of Caucasian decent. Since the identification of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) gene in 1989, and the subsequent improvement in understanding of CF pathophysiology, significant increases in life-expectancy have followed. Initially this was related to improvements in the management and systems of care for treating the various affected organ systems. These cornerstone treatments are still essential for CF patients born today. However, over the last decade, the major advance has been in therapies that target the resultant genetic defect: the dysfunctional CFTR protein. Small molecule agents that target this dysfunctional protein via a variety of mechanisms have led to lung function improvements, reductions in pulmonary exacerbation rates and increases in weight and quality-of-life indices. As more patients receive these agents earlier and earlier in life, it is likely that general CF care will increasingly pivot around these specific therapies, although it is also likely that effects other than those identified in the initial trials will be discovered and need to be managed. Despite great excitement for modulator therapies, they are unlikely to be suitable or available for all; whether this is due to a lack of availability for specific CFTR mutations, drug-reactions or the health economic set-up in certain countries. Nevertheless, the CF community must be applauded for its ongoing focus on research and development for this life-limiting disease. With time, personalised individualised therapy would ideally be the mainstay of CF care.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Selective inhibition of intestinal guanosine 3',5'-cyclic monophosphate signaling by small-molecule protein kinase inhibitors

The guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase II (cGKII) serine/threonine kinase relays signaling through guanylyl cyclase C (GCC) to control intestinal fluid homeostasis. Here, we report the discovery of small-molecule inhibitors of cGKII. These inhibitors were imidazole-aminopyrimidines, which blocked recombinant human cGKII at submicromolar concentrations but exhibited comparatively little activity toward the phylogenetically related protein kinases cGKI and cAMP-dependent protein kinase (PKA). Whereas aminopyrimidyl motifs are common in protein kinase inhibitors, molecular modeling of these imidazole-aminopyrimidines in the ATP-binding pocket of cGKII indicated an unconventional binding mode that directs their amine substituent into a narrow pocket delineated by hydrophobic residues of the hinge and the αC-helix. Crucially, this set of residues included the Leu-530 gatekeeper, which is not conserved in cGKI and PKA. In intestinal organoids, these compounds blocked cGKII-dependent phosphorylation of the vasodilator-stimulated phosphoprotein (VASP). In mouse small intestinal tissue, cGKII inhibition significantly attenuated the anion secretory response provoked by the GCC-activating bacterial heat-stable toxin (STa), a frequent cause of infectious secretory diarrhea. In contrast, both PKA-dependent VASP phosphorylation and intestinal anion secretion were unaffected by treatment with these compounds, whereas experiments with T84 cells indicated that they weakly inhibit the activity of cAMP-hydrolyzing phosphodiesterases. As these protein kinase inhibitors are the first to display selective inhibition of cGKII, they may expedite research on cGMP signaling and may aid future development of therapeutics for managing diarrheal disease and other pathogenic syndromes that involve cGKII.

Elevated Mirc1/Mir17-92 cluster expression negatively regulates autophagy and CFTR (cystic fibrosis transmembrane conductance regulator) function in CF macrophages

Cystic fibrosis (CF) is a fatal, genetic disorder that critically affects the lungs and is directly caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, resulting in defective CFTR function. Macroautophagy/autophagy is a highly regulated biological process that provides energy during periods of stress and starvation. Autophagy clears pathogens and dysfunctional protein aggregates within macrophages. However, this process is impaired in CF patients and CF mice, as their macrophages exhibit limited autophagy activity. The study of microRNAs (Mirs), and other noncoding RNAs, continues to offer new therapeutic targets. The objective of this study was to elucidate the role of Mirs in dysregulated autophagy-related genes in CF macrophages, and then target them to restore this host-defense function and improve CFTR channel function. We identified the Mirc1/Mir17-92 cluster as a potential negative regulator of autophagy as CF macrophages exhibit decreased autophagy protein expression and increased cluster expression when compared to wild-type (WT) counterparts. The absence or reduced expression of the cluster increases autophagy protein expression, suggesting the canonical inverse relationship between Mirc1/Mir17-92 and autophagy gene expression. An in silico study for targets of Mirs that comprise the cluster suggested that the majority of the Mirs target autophagy mRNAs. Those targets were validated by luciferase assays. Notably, the ability of macrophages expressing mutant F508del CFTR to transport halide through their membranes is compromised and can be restored by downregulation of these inherently elevated Mirs, via restoration of autophagy. In vivo, downregulation of Mir17 and Mir20a partially restored autophagy expression and hence improved the clearance of Burkholderia cenocepacia. Thus, these data advance our understanding of mechanisms underlying the pathobiology of CF and provide a new therapeutic platform for restoring CFTR function and autophagy in patients with CF.

Epidemiology and genetics of cystic fibrosis in Asia: In preparation for the next-generation treatments

Cystic fibrosis (CF) in the Asian population is less frequently reported due to under-diagnosis and lack of centralized CF patient registries. Clinical studies on CF cases from Asia have documented a severe course of the disease. The spectrum of the cystic fibrosis transmembrane conductance regulator (CFTR) variants in this population is quite heterogeneous. In total, 166 variants have been reported on approximately 3700 Asian CF chromosomes. The frequency of F508del among Asians is low compared with Caucasians. Recent in vitro studies have shown promise of small molecule correction and potentiation of 45 different CFTR variants. Of these variants, 16 (including G551D and F508del) have also been observed among Asian CF individuals. We suggest undertaking molecular studies extensively to annotate CFTR variants that will help Asian CF individuals to benefit from the precision medicine gaining momentum in the Western countries.

Intestinal current measurement versus nasal potential difference measurements for diagnosis of cystic fibrosis: a case-control study

Background

Nasal potential difference (NPD) and intestinal current measurement (ICM) are functional CFTR tests that are used as adjunctive diagnostic tools for cystic fibrosis (CF). Smoking has a systemic negative impact on CFTR function. A diagnostic comparison between NPD and ICM and the impact of smoking on both CFTR tests has not been done.

Methods

The sweat chloride test, NPD, and ICM were performed in 18 patients with CF (sweat chloride >60 mmol/l), including 6 pancreatic sufficient (PS) patients, and 13 healthy controls, including 8 smokers. The NPD CFTR response to Cl-free and isoproterenol perfusion (Δ0Cl^- + Iso) was compared to the ICM CFTR response to forskolin/IBMX, carbachol, and histamine (ΔI_{sc, forskolin/IBMX+ carbachol+histamine}).

Results

The mean NPD CFTR response and ICM CFTR response between patients with CF and healthy controls was significantly different (p <0.001), but not between patients with CF who were PS and those who were pancreatic insufficient (PI). Smokers have a decreased CFTR response measured by NPD (p = 0.049). For ICM there is a trend towards decreased CFTR response (NS). Three healthy control smokers had NPD responses within the CF-range. In contrast to NPD, there was no overlap of the ICM response between patients with CF and controls.

Conclusions

ICM is superior to NPD in distinguishing between patients with CF who have a sweat chloride > 60 mmol/l and healthy controls, including smokers. Neither NPD nor ICM differentiated between patients with CF who were PS from those who were PI. Smoking has a negative impact on CFTR function in healthy controls measured by NPD and challenges the diagnostic interpretation of NPD, but not ICM.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2466-14-156) contains supplementary material, which is available to authorized users.

An ancient founder mutation in PROKR2 impairs human reproduction

Congenital gonadotropin-releasing hormone (GnRH) deficiency manifests as absent or incomplete sexual maturation and infertility. Although the disease exhibits marked locus and allelic heterogeneity, with the causal mutations being both rare and private, one causal mutation in the prokineticin receptor, PROKR2 L173R, appears unusually prevalent among GnRH-deficient patients of diverse geographic and ethnic origins. To track the genetic ancestry of PROKR2 L173R, haplotype mapping was performed in 22 unrelated patients with GnRH deficiency carrying L173R and their 30 first-degree relatives. The mutation's age was estimated using a haplotype-decay model. Thirteen subjects were informative and in all of them the mutation was present on the same ~123 kb haplotype whose population frequency is ≤10%. Thus, PROKR2 L173R represents a founder mutation whose age is estimated at approximately 9000 years. Inheritance of PROKR2 L173R-associated GnRH deficiency was complex with highly variable penetrance among carriers, influenced by additional mutations in the other PROKR2 allele (recessive inheritance) or another gene (digenicity). The paradoxical identification of an ancient founder mutation that impairs reproduction has intriguing implications for the inheritance mechanisms of PROKR2 L173R-associated GnRH deficiency and for the relevant processes of evolutionary selection, including potential selective advantages of mutation carriers in genes affecting reproduction.

Up-regulated cystic fibrosis transmembrane conductance regulator after anal stenosis in rats

BACKGROUND

The pathophysiology of fecal incontinence from fecal impaction and rectal distension is poorly understood. We hypothesize that fecal impaction elicits up-regulation of cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated mucosal chloride channel.

METHODS

The anus was ligated to produce 75% stenosis in rats. Controls received ligation without inducing stenosis. 24 to 48 hours after ligation the colon was removed. Mucosal short-circuit current was measured by Ussing chamber. Western blot analysis was used to detect CFTR expression in the colonic mucosa. Ligated rats failed to defecate, whereas control rats stooled normally.

RESULTS

Ligated colons were markedly stool filled and dilated. Water content of feces was significantly increased to 66.5% ± 1.1% (P < .01, n = 12) 24 hours after ligation, vs controls (49.5 ± 5.2%, n = 12). Baseline short-circuit current was significantly increased in the distal (78.8 ± 7.4 μA/cm(2), n = 8, P < .01) and mid colon (24.5 ± 2.5 μA/cm(2), n = 8, P < .05) 24 hours after ligation, compared to control rats (12.5 ± 3.2 μA/cm2, n = 8). CFTR expression was significantly increased 24 hours after ligation in the mid and distal colon.

CONCLUSION

We observe that fecal impaction from anal ligation induces early compensatory up-regulation of CFTR, altering function from net absorption to net secretion in the mid and distal colon.

The role played by natural selection on Mendelian traits in humans

Detecting whether and how natural selection has targeted regions of the human genome represents a complementary strategy for identifying functionally important loci and variants involved in disease resistance and adaptation to the environment. In contrast with most complex diseases or traits, the genetic architecture of most Mendelian traits is relatively well established. Most mutations associated with Mendelian disease-related traits are highly penetrant and kept at low population frequencies because of the effects of purifying selection. However, this is not always the case. Here, we review several examples of Mendelian mutations-associated with various disease conditions or other traits of anthropological interest-that have increased in frequency in the human population as a result of past positive selection. These examples clearly illustrate the value of a population genetics approach to unravel the biological mechanisms that have been central to our past and present survival against the selective pressures imposed by diseases and other environmental factors.

Common CFTR gene variants influence body composition and survival in rural Ghana

Various studies in mice have found support for the hypothesis that heterozygous carriers of cystic fibrosis transmembrane conductance regulator (CFTR) mutations have an increased resistance to fatal infection compared to both homozygous mutation carriers and non-carriers, while in humans such evidence is scarce. In this study, we assessed the CFTR heterozygotes survival advantage hypothesis in a contemporary rural population that lives under adverse environmental conditions in the Upper-East region of Ghana. We genotyped 30 SNPs throughout the CFTR gene in 4,230 participants and tested their influence on survival and on body composition in the population at large. With a sliding-window haplotype analysis, we identified a set of six common haplotypes that influenced survival probabilities (global p = 6.00 x 10(-05)). Individual haplotype analyses revealed two haplotypes of specific interest. One of these haplotypes was enriched (p = 0.003), whereas the other was depleted (p = 0.041) among people of old age (> or = 65 years) compared to young study participants (< or = 5 years). In addition, children (n = 474) carrying the latter haplotype had lower body weight (p (trend) = 0.020) and height (p (trend) = 0.010) compared to non-carriers. For all these analyses, similar associations for heterozygous and homozygous CFTR haplotype carriers were observed, revealing an additive effect of haplotype alleles. In conclusion, we identified common haplotypes in the CFTR gene that influence survival and body composition in the population at large with no evidence for heterozygote advantage.

The great opportunity: Evolutionary applications to medicine and public health

Evolutionary biology is an essential basic science for medicine, but few doctors and medical researchers are familiar with its most relevant principles. Most medical schools have geneticists who understand evolution, but few have even one evolutionary biologist to suggest other possible applications. The canyon between evolutionary biology and medicine is wide. The question is whether they offer each other enough to make bridge building worthwhile. What benefits could be expected if evolution were brought fully to bear on the problems of medicine? How would studying medical problems advance evolutionary research? Do doctors need to learn evolution, or is it valuable mainly for researchers? What practical steps will promote the application of evolutionary biology in the areas of medicine where it offers the most? To address these questions, we review current and potential applications of evolutionary biology to medicine and public health. Some evolutionary technologies, such as population genetics, serial transfer production of live vaccines, and phylogenetic analysis, have been widely applied. Other areas, such as infectious disease and aging research, illustrate the dramatic recent progress made possible by evolutionary insights. In still other areas, such as epidemiology, psychiatry, and understanding the regulation of bodily defenses, applying evolutionary principles remains an open opportunity. In addition to the utility of specific applications, an evolutionary perspective fundamentally challenges the prevalent but fundamentally incorrect metaphor of the body as a machine designed by an engineer. Bodies are vulnerable to disease - and remarkably resilient - precisely because they are not machines built from a plan. They are, instead, bundles of compromises shaped by natural selection in small increments to maximize reproduction, not health. Understanding the body as a product of natural selection, not design, offers new research questions and a framework for making medical education more coherent. We conclude with recommendations for actions that would better connect evolutionary biology and medicine in ways that will benefit public health. It is our hope that faculty and students will send this article to their undergraduate and medical school Deans, and that this will initiate discussions about the gap, the great opportunity, and action plans to bring the full power of evolutionary biology to bear on human health problems.

Cystic Fibrosis: Lessons from the Sweat Gland

Lessons from the sweat gland on cystic fibrosis (CF) began long before modern medicine became a science. In European folklore, the curse that “a child that taste salty when kissed will soon die” (Alonso y de los Ruyzes de Fonteca J. Diez Previlegios para Mugeres Prenadas. Henares, Spain, 1606) has been taken by many as a direct reference to cystic fibrosis [Busch R. Acta Univ Carol Med (Praha) 36: 13–15, 1990]. The high salt concentration in sweat from patients with CF is now accepted as almost pathognomonic with this fatal genetic disease, but the earliest descriptions of cystic fibrosis as a disease entity did not mention sweat or sweat glands (Andersen DH. Am J Dis Child 56: 344–399, 1938; Andersen DH, Hodges RG. Am J Dis Child 72: 62–80, 1946). Nonetheless, defective sweating soon became an inseparable, and major, component of the constellation of symptoms that diagnose “cystic fibrosis” (Davis PB. Am J Respir Crit Care Med 173: 475–482, 2006). The sweat gland has played a foremost role in diagnosing, defining pathophysiology, debunking misconceptions, and increasing our understanding of the effects of the disease on organs, tissues, cells, and molecules. The sweat gland has taught us much.

Potential role for the common cystic fibrosis DeltaF508 mutation in Crohn's disease

BACKGROUND

Inflammatory bowel disease (IBD) is an epithelial barrier disease that is thought to result from a dysregulated interaction with bacteria in the intestine of genetically predisposed individuals. The cystic fibrosis transmembrane conductance regulator (CFTR), which is mutated in the autosomal recessive disease cystic fibrosis, modulates gut permeability, mucus production, and epithelial interactions with bacteria. The cystic fibrosis DeltaF508 mutation is commonly found in the general population and has been shown to result in a reduced number of CFTR molecules at the surface of epithelial cells. Given the important biological functions of CFTR in the intestine, we tested whether this mutation is of relevance to IBD.

METHODS

Using DNA heteroduplex analysis, we investigated the distribution of DeltaF508 heterozygosity in 2568 subjects from three independent cohorts of Italian, Swedish, and Scottish IBD patients and controls.

RESULTS

In all three cohorts an association between DeltaF508 and Crohn's disease (CD) was observed. Specifically, DeltaF508 heterozygosity was markedly underrepresented in CD patients from Italy and Sweden (P = 0.021 and 0.027 versus controls, respectively), while stratification for disease location revealed an absence of DeltaF508 carriers among Scottish CD patients with right-sided colitis (P = 0.023 versus all other locations).

CONCLUSIONS

DeltaF508 heterozygosity might exert a protective effect in CD.

Evaluating candidate agents of selective pressure for cystic fibrosis

Cystic fibrosis is the most common lethal single-gene mutation in people of European descent, with a carrier frequency upwards of 2%. Based upon molecular research, resistances in the heterozygote to cholera and typhoid fever have been proposed to explain the persistence of the mutation. Using a population genetic model parameterized with historical demographic and epidemiological data, we show that neither cholera nor typhoid fever provided enough historical selective pressure to produce the modern incidence of cystic fibrosis. However, we demonstrate that the European tuberculosis pandemic beginning in the seventeenth century would have provided sufficient historical, geographically appropriate selective pressure under conservative assumptions. Tuberculosis has been underappreciated as a possible selective agent in producing cystic fibrosis but has clinical, molecular and now historical, geographical and epidemiological support. Implications for the future trajectory of cystic fibrosis are discussed. Our result supports the importance of novel investigations into the role of arylsulphatase B deficiency in cystic fibrosis and tuberculosis.

Population genetics models of common diseases

The number and frequency of susceptibility alleles for common diseases are important factors to consider in the efficient design of disease association studies. These quantities are the results of the joint effects of mutation, genetic drift and selection. Hence, population genetics models, informed by empirical knowledge about patterns of disease variation, can be used to make predictions about the allelic architecture of common disease susceptibility and to gain an overall understanding about the evolutionary origins of such diseases. Equilibrium models and empirical studies suggest a role for both rare and common variants. In addition, increasing evidence points to changes in selective pressures on susceptibility genes for common diseases; these findings are likely to form the basis for further modeling studies.

Luminally active, nonabsorbable CFTR inhibitors as potential therapy to reduce intestinal fluid loss in cholera

Enterotoxin-mediated secretory diarrheas such as cholera involve chloride secretion by enterocytes into the intestinal lumen by the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. We previously identified glycine hydrazide CFTR blockers that by electrophysiological studies appeared to block the CFTR anion pore at its lumen-facing surface. Here, we synthesize highly water-soluble, nonabsorbable malondihydrazides by coupling 2,4-disulfobenzaldehyde, 4-sulfophenylisothiocyante, and polyethylene glycol (PEG) moieties to 2-naphthalenylamino-[(3,5-dibromo-2,4-dihydroxyphenyl) methylene] propanedioic acid dihydrazide, and aminoacethydrazides by coupling PEG to [(N-2-naphthalenyl)-2-(2-hydroxyethyl)]-glycine-2-[(3,5-dibromo-2,4-dihydroxyphenyl) methylene] hydrazide. Compounds rapidly, fully and reversibly blocked CFTR-mediated chloride current with Ki of 2-8 microM when added to the apical surface of epithelial cell monolayers. Compounds did not pass across Caco-2 monolayers, and were absorbed by <2%/hr in mouse intestine. Luminally added compounds blocked by >90% cholera toxin-induced fluid secretion in mouse intestinal loops, without inhibiting intestinal fluid absorption. These orally administered, nonabsorbable, nontoxic CFTR inhibitors may reduce intestinal fluid losses in cholera.

Ex vivo CF diagnosis by intestinal current measurements (ICM) in small aperture, circulating Ussing chambers

Intestinal current measurements (ICM) on rectal suction biopsies are a tool for the ex vivo diagnosis of classical and atypical cystic fibrosis (CF). We present the basic ICM protocol, typical tracings and their interpretation. The ICM technique allows the registration of CF-induced changes in electrogenic transepithelial ion transport (Cl-, HCO3-, K+) in a Cl- secretory epithelium, and on the basis of pharmacological criteria, is able to discriminate between CFTR-mediated Cl- secretion and secretion through alternative anion channels. ICM is particularly useful for the classification of individuals with CF-like clinical features with equivocal sweat test values and/or no or one identifiable CFTR mutation.

CFTR Cl- channel function in native human colon correlates with the genotype and phenotype in cystic fibrosis

BACKGROUND & AIMS

Cystic fibrosis (CF) is caused by over 1000 mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and presents with a widely variable phenotype. Genotype-phenotype studies identified CFTR mutations that were associated with pancreatic sufficiency (PS). Residual Cl- channel function was shown for selected PS mutations in heterologous cells. However, the functional consequences of most CFTR mutations in native epithelia are not well established.

METHODS

To elucidate the relationships between epithelial CFTR function, CFTR genotype, and patient phenotype, we measured cyclic adenosine monophosphate (cAMP)-mediated Cl- secretion in rectal biopsy specimens from 45 CF patients who had at least 1 non-DeltaF508 mutation carrying a wide spectrum of CFTR mutations. We compared CFTR genotypes and clinical manifestations of CF patients who expressed residual CFTR-mediated Cl- secretion with patients in whom Cl- secretion was absent.

RESULTS

Residual anion secretion was detected in 40% of CF patients, and was associated with later disease onset (P < 0.0001), higher frequency of PS (P < 0.0001), and less severe lung disease (P < 0.05). Clinical outcomes correlated with the magnitude of residual CFTR activity, which was in the range of approximately 12%-54% of controls.

CONCLUSIONS

Specific CFTR mutations confer residual CFTR function to rectal epithelia, which is related closely to a mild disease phenotype. Quantification of rectal CFTR-mediated Cl- secretion may be a sensitive test to predict the prognosis of CF disease and identify CF patients who would benefit from therapeutic strategies that would increase residual CFTR activity.

Inhibition of neutral sodium absorption by a prostaglandin analogue in patients with cystic fibrosis

BACKGROUND & AIMS

In normal intestine, cyclic nucleotides (adenosine 3',5'-cyclic monophosphate [cAMP], guanosine 3',5'-cyclic monophosphate) and Ca(2+) inhibit neutral sodium absorption. In contrast, in the jejunum of a knockout mouse model of cystic fibrosis (CF), agents that elevate intracellular cAMP levels did not inhibit neutral sodium absorption, suggesting that the antiabsorptive effect of cAMP is dependent on the cystic fibrosis transmembrane conductance regulator (CFTR). The aim of the present study was to determine if a prostaglandin E(1) analogue, which causes elevation of intracellular cAMP and Ca(2+) levels, inhibits neutral sodium absorption in patients with CF in vivo.

METHODS

Electrolyte and water absorption/secretion was measured during steady state perfusion of the jejunum with a balanced electrolyte solution. Patients with CF and healthy subjects were studied under basal conditions and during intraluminal infusion of a prostaglandin E(1) analogue (misoprostol).

RESULTS

The rate of neutral sodium absorption in the basal state was similar in healthy subjects and patients with CF. Prostaglandin infusion markedly reduced neutral sodium absorption in both healthy subjects and patients with CF. Prostaglandin caused high rates of electrolyte and water secretion in healthy subjects but only trivial rates of secretion in patients with CF.

CONCLUSIONS

CFTR mutations causing CF in humans do not prevent prostaglandin E(1) inhibition of neutral sodium absorption, even though these mutations produce a severe defect in prostaglandin-stimulated electrolyte secretion. These findings suggest that an intact antiabsorptive response to either cAMP or Ca(2+) may contribute to the relatively low level of intestinal disease in patients with CF.

Origin and spread of the 1278insTATC mutation causing Tay-Sachs disease in Ashkenazi Jews: genetic drift as a robust and parsimonious hypothesis

The 1278insTATC is the most prevalent beta-hexosaminidase A ( HEXA) gene mutation causing Tay-Sachs disease (TSD), one of the four lysosomal storage diseases (LSDs) occurring at elevated frequencies among Ashkenazi Jews (AJs). To investigate the genetic history of this mutation in the AJ population, a conserved haplotype (D15S981:175-D15S131:240-D15S1050:284-D15S197:144-D15S188:418) was identified in 1278insTATC chromosomes from 55 unrelated AJ individuals (15 homozygotes and 40 heterozygotes for the TSD mutation), suggesting the occurrence of a common founder. When two methods were used for analysis of linkage disequilibrium (LD) between flanking polymorphic markers and the disease locus and for the study of the decay of LD over time, the estimated age of the insertion was found to be 40+/-12 generations (95% confidence interval: 30-50 generations), so that the most recent common ancestor of the mutation-bearing chromosomes would date to the 8th-9th century. This corresponds with the demographic expansion of AJs in central Europe, following the founding of the Ashkenaz settlement in the early Middle Ages. The results are consistent with the geographic distribution of the main TSD mutation, 1278insTATC being more common in central Europe, and with the coalescent times of mutations causing two other LSDs, Gaucher disease and mucolipidosis type IV. Evidence for the absence of a determinant positive selection (heterozygote advantage) over the mutation is provided by a comparison between the estimated age of 1278insTATC and the probability of the current AJ frequency of the mutant allele as a function of its age, calculated by use of a branching-process model. Therefore, the founder effect in a rapidly expanding population arising from a bottleneck provides a robust parsimonious hypothesis explaining the spread of 1278insTATC-linked TSD in AJ individuals.

The phenotypic consequences of CFTR mutations

Cystic fibrosis is a common autosomal recessive disorder that primarily affects the epithelial cells in the intestine, respiratory system, pancreas, gall bladder and sweat glands. Over one thousand mutations have currently been identified in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene that are associated with CF disease. There have been many studies on the correlation of the CFTR genotype and CF disease phenotype; however, this relationship is still not well understood. A connection between CFTR genotype and disease manifested in the pancreas has been well described, but pulmonary disease appears to be highly variable even between individuals with the same genotype. This review describes the current classification of CFTR mutation classes and resulting CF disease phenotypes. Complex disease alleles and modifier genes are discussed along with alternative disorders, such as disseminated bronchiectasis and pancreatitis, which are also thought to result from CFTR mutations.

Abnormal passive chloride absorption in cystic fibrosis jejunum functionally opposes the classic chloride secretory defect

Due to genetic defects in apical membrane chloride channels, the cystic fibrosis (CF) intestine does not secrete chloride normally. Depressed chloride secretion leaves CF intestinal absorptive processes unopposed, which results in net fluid hyperabsorption, dehydration of intestinal contents, and a propensity to inspissated intestinal obstruction. This theory is based primarily on in vitro studies of jejunal mucosa. To determine if CF patients actually hyperabsorb fluid in vivo, we measured electrolyte and water absorption during steady-state perfusion of the jejunum. As expected, chloride secretion was abnormally low in CF, but surprisingly, there was no net hyperabsorption of sodium or water during perfusion of a balanced electrolyte solution. This suggested that fluid absorption processes are reduced in CF jejunum, and further studies revealed that this was due to a marked depression of passive chloride absorption. Although Na+-glucose cotransport was normal in the CF jejunum, absence of passive chloride absorption completely blocked glucose-stimulated net sodium absorption and reduced glucose-stimulated water absorption 66%. This chloride absorptive abnormality acts in physiological opposition to the classic chloride secretory defect in the CF intestine. By increasing the fluidity of intraluminal contents, absence of passive chloride absorption may reduce the incidence and severity of intestinal disease in patients with CF.

Hopi Indians, "cultural" selection, and albinism

The incidence of albinism in Hopi Indians has been estimated as approximately 1 in 200 individuals. It has been suggested that "cultural" selection as the result of a mating advantage of males with albinism has been important in the maintenance of this high incidence. To examine this hypothesis quantitatively, a model that includes male-mating advantage, mutation, and viability selection is analyzed. In order to play an important role in the maintenance of the high incidence of albinism, the necessary mating advantage of males with albinism appears unrealistically high. However, if the extent of viability selection against individuals with albinism is not as large as previously assumed, the necessary amount of mating advantage is not as high. Other related aspects are also discussed here, such as the type of albinism in Hopi Indians and its impact, the conditions for a polymorphism with male-mating advantage and viability selection, and the time necessary to change the incidence of albinism either by the relaxation or institution of male-mating advantage.

Balanced polymorphism selected by genetic versus infectious human disease

The polymorphisms within the human genome include several functional variants that cause debilitating inherited diseases. An elevated frequency of some of these deleterious mutations can be explained by a beneficial effect that confers a selective advantage owing to disease resistance in carriers of such mutations during an infectious disease outbreak. We here review plausible examples of balanced functional polymorphisms and their roles in the defense against pathogens. The genome organization of the chemokine receptor and HLA gene clusters and their influence on the HIV/AIDS epidemic provides compelling evidence for the interaction of infectious and genetic diseases in recent human history.

Frequency of 657del(5) mutation of the NBS1 gene in the Czech population by polymerase chain reaction with sequence specific primers

Nijmegen breakage syndrome is an autosomal recessive chromosomal instability syndrome characterized by microcephaly, immunodeficiency, radiosensitivity, and predisposition to lymphoid malignancy. A truncating deletion [657del(5)] in exon 6 of the nibrin NBS1 gene is the most frequent cause of the syndrome. Slavic populations carry this mutation in a high frequency. Here, we present polymerase chain reaction with sequence specific primers as a method for the detection of Slavic NBS1 mutation and confirm the high carrier frequency in the Czech population (combined frequency from both studies: 1/106, 95% CI = 1/331 to 1/46).

The Vibrio cholerae haemolysin anion channel is required for cell vacuolation and death

Several strains of Vibrio cholerae secrete a haemolytic toxin of 63 kDa, termed V. cholerae cytolysin (VCC). This toxin causes extensive vacuolation and death of cells in culture and forms an anion-selective channel in planar lipid bilayers and in cells. Here, we identify inhibitors of the VCC anion channel and show that the formation of the anion channel is necessary for the development of the vacuoles and for the cell death induced by this toxin. Using markers of cell organelles, we show that vacuoles derive from different intracellular compartments and we identify the contribution of late endosomes and of the trans-Golgi network in vacuole biogenesis.

Can a place of origin of the main cystic fibrosis mutations be identified?

The genetic background of the mutations that most often cause cystic fibrosis (CF) is different from that of non-CF chromosomes in populations of European origin. It is not known whether these haplotype backgrounds could be found at high frequencies in populations in which CF is, at present, not common; such populations would be candidates for the place of origin of CF mutations. An analysis of haplotypes of CF transmembrane conductance regulator, together with their variation in specific CF chromosomes, in a worldwide survey of normal chromosomes shows (1) a very low frequency or absence of the most common CF haplotypes in all populations analyzed and (2) a strong genetic variability and divergence, among various populations, of the chromosomes that carry disease-causing mutations. The depth of the gene genealogy associated with disease-causing mutations may be greater than that of the evolutionary process that gave rise to present-day human populations. The concept of "population of origin" lacks either spatial or temporal meaning for mutations that are likely to have been present in Europeans before the ethnogenesis of present populations; subsequent population processes may have erased the traces of their geographic origin.

Recent origin and spread of a common Lithuanian mutation, G197del LDLR, causing familial hypercholesterolemia: positive selection is not always necessary to account for disease incidence among Ashkenazi Jews

G197del is the most prevalent LDL receptor (LDLR) mutation causing familial hypercholesterolemia (FH) in Ashkenazi Jew (AJ) individuals. The purpose of this study was to determine the origin, age, and population distribution of G197del, as well as to explore environmental and genetic effects on disease expression. Index cases from Israel (n=46), South Africa (n=24), Russia (n=7), The Netherlands (n=1), and the United States (n=1) were enlisted. All trace their ancestry to Lithuania. A highly conserved haplotype (D19S221:104-D19S865:208-D19S413:74) was identified in G197del chromosomes, suggesting the occurrence of a common founder. When two methods were used for analysis of linkage disequilibrium (LD) between flanking polymorphic markers and the disease locus and for the study of the decay of LD over time, the estimated age of the deletion was found to be 20 +/- 7 generations (the 95% confidence interval is 15-26 generations), so that the most recent common ancestor of the mutation-bearing chromosomes would date to the 14th century. This corresponds with the founding of the Jewish community of Lithuania (1338 a.d.), as well as with the great demographic expansion of AJ individuals in eastern Europe, which followed this settlement. The penetrance of mutation-linked severe hypercholesterolemia is high (94% of heterozygotes have a baseline concentration of LDL cholesterol (LDL-C) that is >160 mg/dl), and no significant differences in the mean baseline lipid level of G197del carriers from different countries were found. Polymorphisms of apolipoprotein E and of scavenger-receptor class B type I were observed to have minor effects on the plasma lipid profile. With respect to determinative genetic influences on the biochemical phenotype, there is no evidence that could support the possibility of a selective evolutionary metabolic advantage. Therefore, the founder effect in a rapidly expanding population from a limited number of families remains a simple, parsimonious hypothesis explaining the spread of G197del-LDLR-linked FH in AJ individuals.